Main Group Elements Questions and Answers

Answer: A

Aluminium carbide gives methane, CH₄.

Al₄C₃ is a methanide: it contains isolated C⁴⁻ centres, each of which takes four protons from water.

\[ \begin{aligned} &\text{Al}_{4}\text{C}_{3} + 12\text{H}_{2}\text{O} \\ &\rightarrow 4\text{Al(OH)}_{3} + 3\text{CH}_{4} \end{aligned} \]

Ethyne (C₂H₂) is what an acetylide such as CaC₂, with C₂²⁻ units, gives on hydrolysis. Ethane and ethene are not produced by hydrolysis of any common ionic carbide.

Answer: A

The correct order is PF₃ < AsF₃ < SbF₃ < BiF₃.

Down group 15 the trifluorides change from covalent molecules to ionic solids. PF₃ is a gas at room temperature and AsF₃ a volatile liquid, both held only by weak intermolecular forces. SbF₃ is a solid that melts near 290 °C, and BiF₃, the most ionic, is a high-melting solid.

The rise follows the increasing electropositive (metallic) character of the central atom, which increases the ionic character of the M-F bond. Every other option puts a heavier fluoride below a lighter one, against this trend.

Answer: D

The pair with non-zero dipole moments is XeF₆ and XeO₃.

Use VSEPR shapes. XeF₂ (three lone pairs) is linear and XeF₄ (two lone pairs) is square planar; in both, the bond dipoles cancel, so the dipole moment is zero.

XeO₃ has one lone pair and three bonds, giving a trigonal pyramidal shape like NH₃, so its bond dipoles do not cancel. XeF₆ has six bonds and one lone pair; the lone pair distorts the octahedron, so it too has a small net dipole.

Every other option includes XeF₂ or XeF₄, which are non-polar.

Answer: D

At 300 ℃ the product is borazine, (HBNH)₃.

Diborane and ammonia first combine at low temperature, but on heating the adduct loses hydrogen and the B-N units close into a six-membered ring of alternating boron and nitrogen atoms, the 'inorganic benzene':

\[ \begin{aligned} &3\text{B}_{2}\text{H}_{6} + 6\text{NH}_{3} \\ &\rightarrow 2\text{B}_{3}\text{N}_{3}\text{H}_{6} + 12\text{H}_{2} \end{aligned} \]

The ratio of one diborane to two ammonia is exactly the one this equation uses.

The other options are species met at low temperature. Diborane and ammonia in the cold give the ionic diammoniate of diborane, formed by unsymmetrical cleavage of the B-H-B bridges; ammonia borane, BH₃·NH₃, is the simple Lewis adduct; and [NH₄]⁺[BH₄]⁻ is a salt-like hydride. All of these lose hydrogen on heating, and at 300 ℃ that dehydrogenation ends in the borazine ring.

Answer: A

The catalyst is copper (Cu).

In the Rochow-Müller (direct) process, powdered silicon reacts with methyl chloride at about 300 ℃ in the presence of copper:

\[ \begin{aligned} &\text{Si} + 2\,\text{CH}_{3}\text{Cl} \\ &\xrightarrow{\text{Cu}} (\text{CH}_{3})_{2}\text{SiCl}_{2} \end{aligned} \]

Copper forms a reactive copper-silicon phase on the surface of the silicon, and dimethyldichlorosilane is the main product; it is the starting material for silicone polymers.

Zinc and tin appear in industrial practice only in trace amounts as promoters added to the copper, not as the catalyst itself, and iron is not used as the catalyst, so Zn, Sn and Fe are all wrong.

Answer: C

The molecule with only one lone pair on xenon is XeO₂F₂.

Xenon has 8 valence electrons. Each Xe-F bond uses one of them and each Xe=O bond uses two; whatever is left over sits on xenon as lone pairs.

• XeF₂: 8 − 2 = 6 electrons, three lone pairs (linear).
• XeF₄: 8 − 4 = 4 electrons, two lone pairs (square planar).
• XeO₂F₂: 8 − 4 − 2 = 2 electrons, one lone pair (see-saw).
• XeO₃F₂: 8 − 6 − 2 = 0, no lone pair (trigonal bipyramidal).

So only XeO₂F₂ has exactly one lone pair; the fluorides have more and XeO₃F₂ has none.

Answer: E

The neutral pair is CO and \(N_{2}O\).

Carbon monoxide and dinitrogen oxide react with neither acids nor bases, which makes them neutral oxides (NO is another).

Each other pair contains a non-neutral oxide: \(Na_{2}O\) is basic, \(Cl_{2}O_{7}\) is strongly acidic, \(Al_{2}O_{3}\) is amphoteric, and \(As_{2}O_{3}\) is amphoteric (mainly acidic).

Answer: C

The keyed reaction is \(Fe + 2HCl \to FeCl_{2} + H_{2}\).

In the laboratory, dihydrogen is prepared by the action of a dilute mineral acid on a reactive metal; granulated zinc with dilute HCl is the textbook choice, and iron with dilute HCl is the same kind of reaction. It runs at room temperature in simple apparatus and gives a steady stream of gas.

The other reactions are not laboratory methods: iron with steam and magnesium with hot water need heating; sodium with water is violently exothermic and dangerous; and calcium with water is less convenient and controlled than a metal-acid reaction.

Answer: E

The alkali metal with the highest first ionization enthalpy is Li.

Down group 1 the atomic radius increases and the outer s electron is held less tightly, so the first ionization enthalpy falls: Li (520) > Na (496) > K (419) > Rb (403) > Cs (376 kJ mol⁻¹).

Lithium, the smallest atom, has the highest value, and caesium the lowest.

Answer: D

Hardness is estimated by titration with \(Na_{2}\)EDTA.

The disodium salt of EDTA forms stable 1:1 complexes with \(Ca^{2+}\) and \(Mg^{2+}\). Titrating a water sample with it (using an indicator such as Eriochrome Black T) measures the total concentration of these ions, which is the hardness.

DMG detects nickel, cupron copper, and \(\alpha\)-nitroso-\(\beta\)-naphthol cobalt; ethylenediamine is a bidentate ligand but is not the titrant for hardness.